High Definition Video On-Demand (HD VOD) systems allow multiple users to simultaneously watch the same or different HD video content. Such systems can provide functionality such as pause, fast-forward, fast-rewind etc. The users' experience is improved when proper lip synchronization is achieved. Lip synchronization refers to matching the visual scene (such as lip movements of a speaker) to the corresponding sound (such as the words spoken by the speaker). Without proper lip synchronization, audio may be heard ahead of or after its corresponding video frame.
To achieve proper lip synchronization, typical HD VOD systems require high performance hardware on a “per stream” basis to support multiple simultaneous streams. In addition, typical HD VOD systems require accurate & high precision timers to manage synchronization between audio and video content. Without such high performance hardware and timers, packets may be dropped or lip synchronization will be lost progressively over time. Lip synchronization may also be lost for every forward and rewind operation. A forward or rewind operation involves jumping ahead, or behind in audio & video content files simultaneously.
However, the usage of a “per stream” high performance hardware pipeline, accurate & precise timers, and complex algorithms needed to achieve proper lip synchronization on a per stream basis can increase the cost of HD VOD hardware significantly. Thus, there is a trade-off between the number of simultaneous streams supported and the hardware required to support the streams.